1. Field of the Invention
This invention relates to an aqueous absorption fluid composition, process and apparatus providing improved water vapor absorption and thermal transfer. Addition of small quantities of certain ethers to an aqueous absorption fluid composition provides improved heat and mass transfer resulting in improved water vapor absorption and enhanced thermal and chemical stability. The compositions of this invention are particularly suited for use in high temperature stages of thermal transfer loops used in absorption refrigeration, chilling, heat pump, energy storage and other thermal transferring applications.
2. Description of Related Art
Aqueous metal halide solutions are well known as refrigerant/absorbent solutions for absorption refrigeration systems as taught by U.S. Pat. No. 3,478,530.
Alcohols have been widely used as additives in small amounts to aqueous refrigerant absorbent solutions for improved heat transfer. Octyl alcohol in LiBr solutions is taught by U.S. Pat. Nos. 3,276,217 and 4,857,222; certain secondary alcohols in LiBr solutions are taught by U.S. Pat. Nos. 3,609,087 and 4,315,411; certain tertiary alcohols are taught by U.S. Pat. No. 3,580,759; and fluoroalcohols are taught by U.S. Pat. No. 3,783,631.
Amines have been used as additives in small amounts to aqueous absorption systems for increased rate of water vapor sorption of working fluid as taught by U.S. Pat. Nos. 5,419,145; 5,577,388 and 5,829,259, for example.
Certain ethers are known as solvents for refrigerant methyl chloride used in absorption refrigeration as taught by U.S. Pat. No. 2,040,905.
The polyfunctional ethers, ethylene glycol monobutyl ether and diethylene glycol monobutyl ether, in amounts of 0.001 to 1.0 percent by weight, in concentrated lithium halide aqueous solutions are known as vapor pressure depressants for use in absorption refrigeration systems, as taught by U.S. Pat. No. 3,553,136.
The use of polyfunctional ether, specifically tetraethylene glycol dimethylether, as an absorbent in conjunction with an azeotropic mixture of trifluoroethanol and water as a cooling medium is taught by Japanese Patent Number 61-14282 to afford a wider temperature range of a cycle than a water/LiBr system in absorption refrigeration.
An absorption fluid of aqueous solutions of metal salts of alkali metal hydroxides, nitrites, and alkaline earth and transition metal hydroxides, halides and thiocyanates and about 10 to about 30 weight percent, based on the metal salt, of an organic compound including ethers, particularly alkaline glycol ethers is taught by U.S. Pat. No. 5,529,709 to provide increased absorbent solubility and vapor pressure reduction.
It is an object of this invention to provide an improved aqueous absorption fluid composition and process for thermal transfer having increased rates of water vapor absorption to result in improved thermal transfer.
It is another object of this invention to provide an improved aqueous absorption fluid composition and process for thermal transfer which allows use of reduced absorber size to obtain a specified thermal transfer.
Yet another object of this invention is to provide an aqueous absorption fluid composition which exhibits high stability to thermal decomposition and chemical reactivity to other components of the composition.
In particular, the general object of the invention can be attained, at least in part and in accordance with one embodiment of the invention, through an absorption fluid composition which includes aqueous refrigerant, at least one metal halide salt absorbent present in an amount to provide a composition useful as a refrigerant/absorbent and at least one monofunctional ether heat and mass transfer additive normally liquid at system operating conditions and present in an amount effective as an absorption promoter.
The invention further comprehends an improvement in a process for thermal transfer using an absorption fluid which includes an aqueous metal halide salt refrigerant/absorbent composition. In accordance with one embodiment of such invention, such improvement relates to obtaining an increased rate of water vapor absorption in the absorption fluid via adding thereto an absorption promoting amount of at least one monofunctional ether heat and mass transfer additive normally liquid at system operating conditions.
The invention still further comprehends an improved apparatus for absorption thermal storage, cooling or heating of the type containing an absorption fluid which includes aqueous refrigerant and at least one metal halide salt absorbent present in an amount to provide a composition useful as a refrigerant/absorbent. In accordance with one preferred embodiment, such improvement relates to the absorption fluid additionally including an absorption promoting amount of at least one monofunctional ether heat and mass transfer additive normally liquid at system operating conditions.
The above objects and other advantages of this invention which will become apparent upon reading this disclosure are achieved by addition of parts per million amounts of certain ethers to aqueous absorption fluids.
As used herein, references to xe2x80x9cmonofunctionalxe2x80x9d ether heat and mass transfer additives are to be understood to refer to such ether additives that contain or include only a single organic functional group, i.e., a single ether chemical group. The term xe2x80x9corganic functional groupxe2x80x9d is not meant to include aliphatic and aromatic hydrocarbon groups such as phenyl, butyl, hexyl and the like. It is to be understood, however, that a monofunctional ether as used herein may, in certain preferred embodiments, be at least partially fluorinated such as to contain one or more fluorine groups.
Further, references herein to xe2x80x9cpolyfunctionalxe2x80x9d ether or other additive material are to be understood to refer to such an additive material which contains or includes two or more organic functional groups, wherein the two or more organic functional groups can be of the same or different type. Examples of polyfunctional additives include: ethylene glycol (i.e., contains or includes two alcohol functional groups), diethylene glycol (ether) (i.e., contains or includes one ether and two alcohol functional groups), ethylene glycol monobutyl ether (i.e., contains or includes one ether and one alcohol functional group), diethylene glycol dibutyl ether, (i.e., contains or includes three ether functional groups), diethylene glycol monobutyl ether acetate (i.e., contains or includes two ether and one ester functional groups), and monophenyl glycol ether acetate (i.e., contains or includes one ether and one ester functional group).
Components in the working fluid of an absorption thermal transfer cycle include chemicals which classified according to their use are defined in the art as follows:
xe2x80x9cRefrigerantxe2x80x9d is the chemical which vaporizes and condenses, or is absorbed, in large volume and the energy associated with this phase change being the essence of the system thermodynamics. In this invention water is the principal refrigerant.
xe2x80x9cAbsorbentxe2x80x9d is the chemical(s) which have relatively low volatilities compared to the refrigerant, and high affinities for the refrigerant. Many suitable absorbents known to the art are suitable for use in this invention, such as those disclosed in U.S. Pat. No. 3,478,530, which is incorporated herein by reference. Lithium, zinc and calcium bromides and chlorides are among suitable absorbents. Lithium bromide compositions are most frequently used in large commercial refrigeration equipment. The properties of the refrigerant and to absorbent(s) together in a refrigerant/absorbent(s) composition define the theoretical limits of the equilibrium thermodynamics of the absorption system. In this invention suitable absorbent(s) are at least one metal salt which is present in an amount sufficient to provide a composition functional as a refrigerant/absorbent(s) composition. Any of the above mentioned salts or combinations of salts, and also salts in combination with other non-interfering absorbents such as glycols or amines are suitable for use in this invention. xe2x80x9cHeat and mass transfer additive,xe2x80x9d an absorption promoter, serving to accelerate the rate of dissolving or absorption of the refrigerant by the absorbent. Added in small quantities, the heat and mass transfer additive does not directly change the system thermodynamics. Suitable heat and mass transfer additives according to this invention include certain ethers. This invention uses certain ethers as heat and mass transfer additives and not as absorbents. The prior art has used polyfunctional ethers, such as glycol ethers, in significantly large concentrations as absorbents. However, glycol ethers in small quantities, as illustrated by Comparative Example XIV, do not serve as heat and mass transfer additives as intended by the present invention. None of the prior art known to the inventor teaches monofunctional ethers as heat and mass transfer additives. Other additives, as pointed out in the Description of Related Art section above, particularly, the addition of certain amines as described in U.S. Pat. No. 5,419,145, which is incorporated herein in its entirety by reference, known for increasing the rate of absorption of the refrigerant into the absorbent to form the refrigerant/absorbent composition may be used in conjunction with the ether heat and mass transfer additives of this invention, as long as they are non-interfering.
xe2x80x9cCorrosion inhibitorsxe2x80x9d for addition to the working fluid compositions are known to the art, such as, for example, salts of molybdate, nitrate, chromate, etc.; bases such as lithium hydroxide used to raise pH; and organic inhibitors, such as, benzotriazole and related compounds. Corrosion inhibitors sometimes interfere with other additives and one of the advantages of the ether heat and mass transfer additives of this invention is that they are significantly less susceptible to interfering reactions with corrosion inhibitors, and in general are more stable in an absorption chiller environment than other additives known to the prior art.
Other additives, such as, for example, crystallization inhibitors and other absorption promoters may be used as long as they do not interfere with the action of the heat and mass transfer additives of this invention.
This invention provides an improved absorption or absorption refrigeration fluid of the type comprising water as the refrigerant and at least one absorbent comprising a metal salt present in an amount sufficient to provide a functional refrigerant/absorbent(s) composition having an amount of monofunctional ether(s) sufficient to provide higher heat and mass transfer resulting in increased rate of thermal transfer of the absorption thermal transfer system.
The metal salt absorbent reduces the vapor pressure of the aqueous refrigerant/absorbent composition sufficiently to provide a composition useful as a refrigerant/absorbent composition. Some suitable salts include at least one cation selected from the group consisting of ammonium, alkali metals, alkaline earth metals, transition metals and mixtures thereof, and at least one anion selected from the group consisting of halide, nitrate, nitrite, thiocyanate and mixtures thereof. Some metal salts useful in this invention include, for example, ammonium or alkali metal, such as lithium, nitrates, nitrites and halides, such as, chlorides, bromides and iodides; alkaline earth metal, such as calcium and magnesium, transition metal, such as iron, cobalt, copper, aluminum and zinc halides and thiocyanates; and mixtures thereof. Particularly suitable for use in this invention are lithium bromide, lithium chloride, lithium iodide, zinc chloride, zinc bromide, zinc iodide, calcium chloride, calcium bromide, calcium iodide, lithium nitrate and mixtures thereof. Particularly suitable combinations include combinations of transition metal or zinc salts, such as zinc bromide, zinc chloride and mixtures thereof, with lithium bromide, lithium chloride and mixtures thereof. Lithium bromide is a particularly useful salt in the refrigerant/absorbent composition of this invention. Suitable initial concentrations of metal salts in the refrigerant/absorbent compositions of this invention are about 30 to about 85 weight percent, and preferably, about 45 to about 75 weight percent.
In accordance with certain preferred embodiments of the invention, absorbents for use in the invention comprise one or more metal halide salts. As will be appreciated, absorption fluid compositions in accordance with the invention may also advantageously include one or more corrosion inhibitors, such as described above, or in the form of one or more metal hydroxides, present in low levels or concentrations. Specific preferred candidates of such metal hydroxide corrosion inhibitors include: lithium hydroxide for those fluids of lithium bromide and lithium hydroxide, zinc hydroxide, or possibly zinc oxide, or some combination thereof for those fluids containing zinc and lithium bromide.
The use of lithium hydroxide and the like to control or assist in controlling corrosion is generally known to those skilled in the art. As will be appreciated, the specific amount used of such corrosion inhibitor can be dependent on various factors such as the presence and amounts of other corrosion inhibitors. In general, the corrosion inhibiting inclusion of lithium hydroxide to a concentrated lithium bromide solution will fall in the range of about 0.002N to about 0.300N lithium hydroxide.
It is to be understood, however, that the broader practice of the invention is not necessarily limited to non-acidic solutions. For example, the ether additives of the invention may, if desired, be employed in conjunction with acidic solutions.
In one embodiment of this invention, a useful refrigerant/absorbent composition comprises the combination of zinc and lithium bromides and a corrosion inhibitor. The combination may have ratios of zinc bromide/lithium bromide of about 3/1 to about 1/3 by weight, particularly suitable ratios being about 1.6 to about 1.9. A suitable source of corrosion inhibitor is lithium hydroxide in an amount of about 0.0005 to about 0.02 gram per gram of total salt.
Suitable monofunctional ethers for use as heat and mass transfer additives for aqueous absorption fluids according to this invention include ethers normally liquid at system operating conditions. Suitable monofunctional ethers include aliphatic and aromatic ethers which do not normally form stable, i.e., irreversible, complexes with the metal salt absorbent in the composition, where irreversible is defined as a complex that remains bound substantially longer than the ligand (water) exchange rate. Particularly, poly ethers based upon short chain glycols, such as those forming 5 to about 8 membered rings with the absorbent cation, are not suitable, as demonstrated by Comparative Example XIV.
Suitable aliphatic monofunctional ethers include straight and branched chain symmetric and asymmetric ethers and cyclic ethers. Cyclic ethers may be especially preferred. Aliphatic ethers for use in the practice of the invention include those having about 4 to about 20 carbon atoms, such as, for example, butyl, pentyl, hexyl, heptyl, octyl, nonyl, decyl, undecyl, dodecyl, tridecyl, tetradecyl, pentadecyl, hexadecyl, heptadecyl, octadecyl, nonadecyl, and eicosyl which may be symmetrical or asymmetrical, branched chain, or cyclic. Cyclic ethers, such as, for example, substituted tetrahydrofurans, are examples of preferred cyclic ethers. Lower molecular weight ethers, 6 carbon atoms and less, are generally not as satisfactory for use because they tend to have too high a volatility, resulting in lower performance at higher concentrations of ether additive. At least partially fluorinated ethers, by which is meant an ether containing one or more fluorine atoms replacing one or more hydrogen atoms in the hydrocarbon, such as fluorinated and perfluorinated cyclic ethers, e.g., perfluoro-2-butyltetrahydrofuran, and fluorinated and perfluorinated ethers having about 8 to about 24 carbon atoms, e.g., perfluoroheptyl ether, perfluorooctyl ether, perfluorononyl ether, perfluorodecyl ether, and the like, may serve to provide good results at lower operating temperatures.
Aromatic monofunctional ethers having about 7 to about 20 carbon atoms are suitable for use in this invention, such as, for example, diphenyl ether, methylphenyl ether (anisole), and dibenzyl ether. Also suitable for use in the practice of the invention are at least partially fluorinated aromatic ethers.
At least partially fluorinated ethers such as the fluorinated and perfluorinated compounds described above, e.g., perfluorinated 2-butyltetrahydrofuran, have been found especially suitable for use as heat and mass transfer additives according to this invention due to their enhancement of thermal stability, such as may be necessary for direct-fired double-effect and triple-effect chillers, especially in mixed lithium-zinc bromide compositions.
The amount of monofunctional ether heat and mass transfer additive needed to obtain maximum results depends upon the combination of the specific heat and mass transfer additive used, the refrigerant/absorbent composition, and the operating conditions. The amount of a specific monofunctional heat and mass transfer additive desired for maximum results can be readily determined by one skilled in the art. Suitable relative amounts of heat and mass transfer additive range from about 5 ppm to about 10,000 ppm, and most preferably about 5 ppm to about 2,000 ppm, where such additive proportions are based on the weight of the total refrigerant/absorbent composition. About 50 ppm to about 500 ppm of the heat and mass transfer additive is preferred in many refrigerant/absorbent compositions. When refrigerant/absorbent compositions having lower volatility are used, less volatile heat and mass transfer additives are needed. Unless a heat and mass transfer additive of sufficiently low volatility is used, suitable upper concentrations of heat and mass transfer additives are about 100 ppm to 500 ppm for hydrocarbon ethers and about 400 ppm to about 2000 ppm for perfluorinated ethers, due to the increase in molecular weight.
The lower molecular weight ethers, 8 carbon atoms and less, such as, anisole or butyl ether, do not do as well when run at high temperatures and absorbent concentrations because they tend to be too volatile. A consequence of excessive heat and mass transfer additive volatility is a drop in performance at higher additive concentrations. Higher molecular weight ethers according to this invention, such as those having about 12 to about 24 carbon atoms, including at least partially fluorinated ethers, such as perfluorooctyl ether, perfluorononyl ether, perfluorodecyl ether, and the like, for example, are especially suitable for use in double-effect machines operating with generator temperatures of about 360xc2x0 F. and triple-effect machines operating with generator temperatures of about 450xc2x0 F.
The refrigerant/absorbent compositions of this invention may also include one or more other heat and mass transfer additives, such as amines as taught by U.S. Pat. Nos. 5,577,388 and 5,829,259 and alcohols.
The refrigerant/absorbent compositions comprising ethers for use as heat and mass transfer additives in this invention may also contain one or more additional optional additives, such as, for example organic crystallization inhibitors, usually having an amino or hydroxyl functional group, corrosion inhibitors, or other additives, as long as they do not significantly interfere with the activity of the ether function.
This invention includes a process and apparatus for absorption thermal storage, cooling or refrigeration using the refrigerant/absorbent composition of this invention. Suitable apparatus and process for use of the refrigerant/absorbent composition of this invention includes absorption chillers and refrigeration systems as disclosed in U.S. Pat. Nos. 4,966,007, 5,038,574 and 5,186,009, thermal energy storage systems as disclosed in U.S. Pat. 4,823,864, as well as multiple effect absorption refrigeration systems, for example, double effect and dual loop systems disclosed in U.S. Pat. Nos. 3,266,266 and 4,542,628 and triple effect systems disclosed in U.S. Pat. Nos. 5,335,515 and 5,390,509. The disclosures in the aforesaid patents are incorporated herein in their entirety, including but not limited to the descriptions of the apparatus and systems disclosed therein. Especially preferred are the single and double effect absorption chiller and refrigeration systems which include those systems in which the single or double effect components are a portion of the system, such as a dual loop triple effect system comprising combined single stage loops as disclosed in U.S. Pat. No. 4,732,008.
The following examples are set forth in specific detail for exemplification of the invention and should not be considered to limit the invention in any way.